I. Field of the Invention
The present invention relates to the field of wireless communications. More particularly, the present invention relates to a novel and improved system for using different wireless signal polarizations to maintain diversity between a repeater and a base station.
II. Description of the Related Art
Wireless communication entails the transmission of electromagnetic waves through free space. In a wireless communications system, a base station receives a wireless signal from a communications device, such as a mobile phone. Some base stations receive diverse versions of the wireless signal by using two antennas. This technique is referred to as receive diversity. Receive diversity improves system performance because one version of the signal may still be available if the other version of the signal fades.
A wireless repeater is sometimes used between the communications device and the base station. The repeater extends the range of the base station by amplifying and re-transmitting the wireless signal between the communications device and the base station. Some repeaters also use receive diversity by receiving two versions of the wireless signal from the communications device. The repeater sums the two versions into a combined signal and transmits the combined wireless signal to the base station.
The current repeater solution is lacking. If the transmitter malfunctions, the wireless signal from the repeater is lost or flawed and user communications are disrupted. The summing of the two versions of the wireless signal in the repeater degrades the Signal-to-Noise Ratio (SNR) by at least three decibels in low SNR conditions. In addition, system performance suffers because the ability to isolate the diverse versions is lost after they are summed. Even if the base station also has receive diversity, each base station antenna will receive the combined wireless signal, and not the original diverse versions received by the repeater.
Multipath signals are different versions of the same wireless signal that are generated by reflections from structures and natural formations. Multipath signals can have phase shifts that cause the signals to cancel each other out at certain locations. The loss of a signal due to the phase cancellation of multipath signals is known as fading. Fading is a serious problem in wireless systems because it disrupts user communications. For example, several multipath copies of a single wireless signal transmitted by a wireless communications device may be generated by reflections from trees and buildings. These multipath copies may combine in the base station and cancel each other out due to phase offset.
The loss of diversity in the repeater has an impact on wireless systems that use Code Division Multiple Access (CDMA). One form of CDMA is specified in the IS-95 standard approved by the Telecommunications Industry Association, but the invention is not restricted to the form of CDMA specified in this particular standard. CDMA systems transmit and receive wireless signals within a single frequency band and use codes to separate the individual signals. In contrast, other systems use frequency and time division to separate the individual signals. CDMA systems have demonstrated clear advantages in the areas of capacity, voice quality, privacy, and cell hand-off.
CDMA systems require power control. The SNR represents the power of a signal relative to the surrounding noise. An adequate SNR must be maintained so that the signal can be separated from the noise. Since CDMA signals are not divided by frequency for time a given link direction, the noise component of the ratio includes all other received CDMA signals. If the power of an individual CDMA signal is too high, it effectively drowns out all other CDMA signals. Thus, power control is used to maintain an equivalent power level for all user signals received at the base station. The power level of these received CDMA signals should be minimized, but still must be strong enough to maintain the appropriate SNR. CDMA systems dynamically control the transmit power of each communications device to maintain the appropriate power level of the received signals at the base station. Dynamic control is applied through open loop and closed loop control techniques that are known in the industry.
The range of the CDMA system is directly related to the common power level of the received signals because each additional signal adds noise to all of the other signals. The user noise component of the SNR is reduced when the average receive power level is lowered. Techniques that decrease CDMA signal power from the communications device directly increase the range of the CDMA system. Receive diversity is one technique used to minimize the required signal power. Lower signal power also lowers the cost of the user communications devices while increasing operational battery life as well as the range.
Unlike other wireless systems, CDMA systems can process multipath signals to provide additional diversity. Unfortunately, multipath signals that are not separated by a sufficient time delay may still cause fading in a CDMA system. Signal power is typically increased to overcome fading, but the increased signal power reduces the range of the system.
Prior CDMA systems have used receive diversity at the repeater and base stations. Unfortunately, the diversity is lost in the repeater because the diverse versions of the signal are summed. This loss of diversity increases the signal power requirement and decreases the range of the CDMA system. The performance of any wireless communications system could be improved if the receive diversity of the repeater is maintained through to the base station. For CDMA systems, the range of the systems can be increased if the signal power can be reduced through improved diversity.
Polarization is a known characteristic of electromagnetic radiation. Polarization refers to electrical field vectors that are perpendicular to the direction of electromagnetic wave propagation. For wireless signals, the polarization vectors are typically linear or circular or, in general, elliptical when viewed in the time domain. For circular and elliptical polarization, the electric field vector traces a circle or, in general, an ellipse in the time domain as the wave propagates. The direction of rotation can be either right-hand or left-hand relative to the direction of propagation. Thus, two common forms of polarization are right-hand circular and left-hand circular. Linear polarization vectors are static in the time domain. Linear polarization vectors are further characterized as horizontal, vertical, right-slant, or left-slant. The vectors in horizontal and vertical polarization are aligned with the horizontal and vertical axis respectively. The right-slant and left-slant are offset from the vertical to the right and left respectively. Thus, two more common forms of polarization are linear right-slant and linear left-slant.
Two signals have orthogonal polarization states if one polarization state contains no components of the other polarization state. Orthogonal polarization is easy to visualize with linear states. Horizontal and vertical states are orthogonal. Linear states at tilt angles of 45 degrees and 135 degrees are also orthogonal. The simplest representation for visualizing orthogonal wave states is the Poincarxc3xa9 Sphere. Those knowledgeable in the art, specifically in the mathematics of electromagnetic wave polarization, understand that every possible polarization state for a completely polarized wave can be assigned to a point on the surface of the Poincarxc3xa9 Sphere. Polarization states are orthogonal if represented by opposite points on the Poincarxc3xa9 Sphere. Polarization is discussed further in Polarization in Electromagnetic Systems by Warren L. Stutzman, published by Artech House, Norwood, Me., 1993, and ISBN 0-89006-508-X; and in Satelite Communications by Timothy Pratt and Charles W. Bostian, published by John Wiley and Sons, New York, N.Y., 1986 and ISBN 0-471-87837-5.
Some proposed satellite systems transmit microwave signals that use different signal polarizations for different groups of users. The proposed system does not use different signal polarizations to provide signal diversity for the same user. The use of different signal polarizations has not been extended to the link between a wireless repeater and a wireless base station to provide diverse versions of the same wireless signal.
The present invention is a novel and improved system that maintains receive diversity between the wireless repeater and the wireless base station. The repeater does not sum the diverse versions of the wireless signal, but transmits them to the base station using diverse signal polarities. The base station uses the diverse polarities to receive and isolate the diverse versions of the wireless signal and maintain the receive diversity of the repeater.
The use of diversely polarized wireless signals improves overall system diversity between the repeater and base station. The improved diversity lowers signal power requirements and increases the capacity and range of wireless base stations that use CDMA. The diversely polarized versions of the wireless signal can be isolated so that processing of the diversity signals in the base station is optimized. In addition to these advantages, the repeater does not incur the three-decibel SNR loss because it does not sum the diverse versions of the signal. Also, reduced transmit power at the user communications device lowers the cost of the device while increasing operational battery life and range.
The invention includes methods and systems for use in a wireless communications system where a communications device transmits a wireless signal. A repeater system receives a first version of the wireless signal using a first receiver system and receives a second version of the wireless signal using a second receiver system. The repeater system transmits a third version of the wireless signal that has a first polarization and that is based on the first version of the wireless signal. The repeater system transmits a fourth version of the wireless signal that has a second polarization and that is based on the second version of the wireless signal.
A base station system receives the third version of the wireless signal using the first polarization and receives the fourth version of the wireless signal using the second polarization. The base station system receives the third version of the wireless signal and the fourth version of the wireless signal and processes these versions in the conventional manner. The base station may also concurrently process signals from other communications devices that do not use the repeater system.
In some embodiments of the invention, the wireless signal is a CDMA signal. The first polarization and the second polarization could be orthogonal to one another, such as linear right-slant and linear left-slant that are off-set by 45 degrees from the vertical. The first polarization and the second polarization could alternatively be right-hand circular and left-hand circular respectively. The repeater""s two transmitter systems could be comprised of a single antenna structure that has two distinct polarization characteristics, and the base station""s two receiver systems could also be comprised of a single antenna structure that has the same two distinct polarization characteristics.